The invention relates to a method and arrangement for managing a service in a mobile communications system, a mobile station and an intelligent module of a mobile station. The invention is advantageously applied to a location method for a mobile station. The method can be applied to analog and digital mobile communications systems. An example of such a system is the digital, time division based GSM (Global System for Mobile Communications).
Location information of a mobile station can be used for many purposes:
pricing of calls can be performed according to the location of a mobile station, whereby calls made from the home area, for example, can be cheaper;
when an emergency call is made from a mobile station, it is possible to determine the location of the calling mobile station (this function will be required by the authorities in some countries);
the user of a mobile station may need information about his/her location e.g. when traveling,
the authorities can use the location information to locate a stolen mobile station or to trace a missing person, for example.
As the location process involves data traffic in the system, it is necessary that the operator be able to charge for the location service according to use. In addition, the location process has to be secure so that the location information cannot be falsified by the user e.g. in order to be allowed a home area discount while elsewhere than in the home area. Furthermore, the data may be confidential as it is not necessarily in the interests of the operator to impart system-related information to a third party.
The Global Positioning System (GPS) based on the use of signals received from satellites is widely used for location purposes. This arrangement would require that a GPS receiver be added to mobile stations, which would result in considerable additional costs. Therefore, in a location system intended for all mobile phones it is preferable to utilize the cellular system""s own signals transferred between the mobile station and base stations to locate a mobile station, even though some mobile station models may also include a GPS receiver.
A method is known from publication [1] WO 92/05672 wherein the distance of a mobile station from a base station is determined on the basis of the propagation delay of a signal transmitted between the mobile station and the base station. FIG. 1 illustrates the operating principle of such a method. By measuring at a base station 1 the propagation delay of a signal sent by a mobile station 14, it is obtained an estimated distance d1 between the mobile station and the base station. Because of a certain measuring accuracy of the distance measurement, the assumed location of the mobile station is a ring-shaped area the width of which depends on the time delay measuring accuracy. A corresponding propagation delay measurement can be performed for a signal between the mobile station and other base stations. The result is one ring-shaped location area per each base station involved in the measurement. In the situation illustrated by FIG. 1, the propagation delays between the mobile station and base station 11, mobile station and base station 12, and mobile station and base station 13 produce estimated distances d1, d2 and d3 which correspond to the ring-shaped areas 16, 17 and 18. The rings intersect at area 19, which is the mobile station location area obtained from the three propagation delay measurements. Thus, the location of the mobile station can be determined to be an area the order of dimension of which equals the measuring accuracy. In the method described above, locating is substantially carried out by the system, and no special functions are needed in mobile stations to realize the measurement.
A disadvantage of the method based on the propagation delay measured from a signal between a mobile station and base stations is that the propagation delay cannot be easily measured at such an accuracy that the accuracy of the mobile station location information would be adequate for all the above-mentioned applications. In addition, the mobile station has to establish an uplink connection with every base station for which the propagation delay is to be measured. If the location information has to be continuously updated, the data traffic related to the location information puts a considerable load on the connection capacity of the mobile communications system. Furthermore, the high volume of data communications makes the measuring slow. It is also a problem of the method that errors in the absolute timing accuracy of the mobile station produce errors in the location result obtained.
FIG. 2 illustrates the operating principle of a time difference based measurement in which the measurement can be performed by the mobile station or by the system. Let the mobile station receive signals from a plurality of base stations, in the case depicted by FIG. 2 from three base stations BTS1, BTS2 and BTS3. An estimate for the location of the mobile station is determined by means of the observed time difference (OTD) between the signals received from the base stations, whereby it is possible to calculate by means of the time difference between signals received from two base stations the difference d1-d2 between the distance d1 between the mobile station and a first base station and the distance d2 between the mobile station and a second base station. Then those potential location points of the mobile station in which the value of the distance difference equals d1-d2 constitute a hyperbola-shaped curve, which thus represents the potential location points of the mobile station. In FIG. 2, this curve is depicted by a dashed line. Since the measurement result has a certain error margin, the location area of the mobile station is in reality a band between two hyperbolas, the width of the band depending on the error margin of the measurement result. When signals are received from at least three base stations BTS1, BTS2 and BTS3, the result consists of a plurality of location areas A12, A13 and A23 such that the mobile station is located at the intersection AMS of those areas. Determining a restricted location area requires time difference measurement for signals received from at least three base stations unless other methods such as propagation delay measurement are used in addition to the time difference measurement. If other additional methods are used, it is possible to use the time difference measured for signals received from only two base stations. So, when using the measurement principle depicted in FIG. 2, the location of a mobile station can be calculated either by the mobile station or by the system.
From patent document [2] FI 101445 it is known a solution in which a mobile station measures the time differences of signals received from base stations and transmits the measured time difference data to the mobile communications system. The mobile location center in the mobile communications system calculates the location of the mobile station on the basis of the measured time difference data and base station location coordinates and base station real time difference (RTD) data. This solution has the disadvantage that time difference data have to be transmitted from the mobile station to the system, which puts a load on the data transfer capacity in the uplink direction.
In the solutions disclosed in documents [1] and [2], the location of a mobile station is calculated by the cellular system. Then the mobile station location information must be transmitted from the system to the mobile station in a special message if the application requiring the location information is in the mobile station. If the user needs the location information continually, a lot of data transfer will occur in the downlink direction, too. Another disadvantage is that the user receives no real-time location information because the location information has to be first calculated by the system.
A solution is known from document [3] EP 398773 wherein a mobile station receives from a mobile communications system information about timing differences and location coordinates of base stations situated in the area around the mobile station. Then the mobile station measures the time differences of the signals received from said base stations and determines the location of the mobile station on the basis of the measured time differences, base station timing differences and location coordinates, using a location algorithm stored in the mobile station. Said document does not describe in detail how the signalling in the method would be implemented in a mobile communications system. A disadvantage of this solution is, however, that the operator cannot charge the user for the location service because the mobile station can perform the location independently on the basis of signals sent by base stations. Another disadvantage of the solution is that the user can falsify the location information sent to the system, so that the location method is not reliable enough for many applications.
So, the use of location information of a mobile station has many applications, but the features in the prior-art solutions do not meet all the requirements of the applications.
An object of the invention is to provide a solution for the management of a service in a mobile communications system, eliminating the above-mentioned disadvantages related to the prior art. By means of this solution it is possible to realize a method for determining the location information of a mobile station and a mobile station that meet the requirements concerning the chargeability, reliability and confidentiality of the location service.
An idea of the invention is that the data required for a service in a mobile station are transferred in encrypted form to several mobile stations at the same time. The encryption is based on the use of a decryption key which is sent to the mobile stations individually.
Applied to locating a mobile station this means that the position of the mobile station is determined in the mobile station on the basis of the timing difference between signals received from at least two (preferably at least three) base stations and using the location information of said base stations. Location information means base station position coordinates, real time difference (RTD) data and other base station related data that are needed to determine the location of a mobile station. The base station location information is transferred to the mobile station in encrypted form so that it can be used only by mobile stations to which the operator has delivered the decryption key needed for the decryption of the location information. The decryption key is preferably transferred individually and in encrypted form to the mobile stations. The decryption key may be changed, in which case the new decryption key is transferred to a mobile station advantageously in connection with location update, which is a procedure used in GSM.
In the transfer of the decryption key and location information, decryption advantageously takes place in the intelligent module (say, smart card) so that also the location information can be computed using an algorithm stored in the intelligent module. This results in both service chargeability and data confidentiality, provided that the decryption key is encrypted and the decryption of the decryption key takes place in the intelligent module. If only chargeability is required, decryption and location information calculation may be performed elsewhere in the mobile station than in the intelligent module. In addition, the location information sent to the system may be encrypted in the intelligent module prior to transmission if the application requiring the mobile station location information is in the system.
The invention facilitates real-time, continuous location calculation in speech, data and standby modes because the location of the mobile station is calculated in the mobile station. Since the location information is encrypted, it is possible to make the location service available to only those who specifically order the service, and the use of the service is chargeable. Charging may be based on the delivery of decryption keys or it may be in the form of monthly billing, for example. A significant advantage of the solution according to the invention is also that a user, for instance, cannot falsify the data used in the location process, nor the location information sent to the system if the only place where these data are in a non-encrypted form is the smart card of the mobile station.
The method for managing a service, in which information is transferred to a plurality of mobile stations simultaneously on one and the same transmission channel in a mobile communications system, is characterized in that said information transferred simultaneously to a plurality of mobile stations is encrypted and that the encryption is realized using a decryption key which is transferred separately to each mobile station.
The method according to the invention for locating a mobile station, wherein
the observed time difference between signals received from at least two base stations is measured in the mobile station,
the location coordinates of said at least two base stations are transferred to the mobile station, and
the location coordinates of the mobile station are computed in the mobile station on the basis of said time difference and location coordinates of said at least two base stations, is characterized in that
said base station location coordinates are transferred to the mobile station in encrypted form.
The mobile station location system according to the invention, comprising a mobile communications system which includes a plurality of base stations and mobile stations, wherein
the observed time difference between signals received from at least two base stations is measured in the mobile station,
the location coordinates of said at least two base stations are transferred to the mobile station, and
the location coordinates of the mobile station are computed in the mobile station on the basis of said time difference and location coordinates of said at least two base stations, is characterized in that
arrangements are made for said base station location coordinates to be transferred to the mobile station in encrypted form.
The mobile station according to the invention which comprises means for receiving on a broadcast control channel information related to a service is characterized in that it also comprises means for decrypting said information by means of a decryption key, and means for receiving said decryption key on a channel assigned to said mobile station.
The mobile station according to the invention which for the purpose of determining the location of the mobile station comprises
means for receiving signals from at least two base stations,
means for determining the observed time difference between received signals,
means for transferring the location coordinates of said at least two base stations to the mobile station, and
means for determining the mobile station location coordinates on the basis of said time difference and location coordinates of said at least two base stations, is characterized in that the mobile station also comprises means for decrypting said base station location coordinate information.
The mobile intelligent module according to the invention which comprises means for receiving encrypted information related to a service from a mobile station is characterized in that the intelligent module also comprises means for decrypting said information using a decryption key and means for receiving said decryption key from a mobile station.
A mobile smart card according to the invention is characterized in that the smart card comprises means for generating a decryption key used in the encryption of base station location coordinate data on the basis of information received from a base station.
Preferred embodiments of the invention are disclosed in the dependent claims.